Field of the Invention
The present invention relates to an electrochromic element, and an optical filter, a lens unit, an image pickup apparatus, and a window member each using the element.
Description of the Related Art
An electrochromic (hereinafter sometimes abbreviated as “EC”) element that uses an EC material as a substance that undergoes changes in optical absorption properties (absorption wavelength and absorbance) through an electrochemical redox reaction is applied to a display apparatus, a variable reflectance mirror, a variable transmission window, and the like. The absorption wavelength of an organic electrochromic compound out of the EC materials can be designed and changed, and hence the compound can achieve high coloring/decoloring contrast. Accordingly, active development of the compound has been performed.
One of the greatest challenges posed by such EC element is the suppression of changes in optical characteristics with a lapse of time. In U.S. Pat. No. 6,188,505, there is a disclosure that a material that is more easily oxidized than a non-EC anodic EC material and a material that is more easily reduced than a cathodic EC material are used in a complementary EC element obtained by dissolving an EC material in an electrolyte. Those materials are hereinafter referred to as “redox buffers.”
In the EC element disclosed in U.S. Pat. No. 6,188,505, an oxidized form and a reduced form of the redox buffers are more stable than an oxidized form as a colored form of the anodic EC material and a reduced form as a colored form of the cathodic EC material, respectively. Therefore, even when charge imbalance occurs in a decoloring operation, insofar as the amounts of charge of the redox buffers can cover, generation of the oxidized form and the reduced form of the corresponding redox buffers are dominant over remaining of the colored forms of the EC materials. The redox buffers are non-EC materials, and thus, even when the oxidized form and the reduced form of the redox buffers are generated, the redox reactions thereof do not affect the transmittance of light. In other words, the redox buffers add a charge balance region in which the color does not fluctuate so that the charge imbalance of the EC element does not directly result in a decoloring failure.
However, in U.S. Pat. No. 6,188,505, the redox buffers are more easily oxidized than the anodic EC material or more easily reduced than the cathodic EC material, and thus, are more liable to react in terms of the potential than the EC materials. Therefore, in a normal coloring operation of the EC element, the redox buffers react ahead of (at least equivalent to) the EC materials. As a result, there are problems in that, as compared to a case where the redox buffers are not used, a current that does not contribute to the coloring unnecessarily flows to increase the power consumption and to reduce the response speed.
Further, even when the redox buffers are used as in U.S. Pat. No. 6,188,505, charge imbalance between display electrodes is not eliminated. Specifically, only the colored forms of the EC materials are reduced (instead, the oxidized/reduced forms of the redox buffers that are not colored or decolored are generated), and the charge balance between the display electrodes is not affected. When charge imbalance occurs in a complementary EC element, the ratio of the colored form of the anodic EC material to the colored form of the cathodic EC material changes.
Specifically, there occurs a situation in which the ratio of the colored form of a material opposite in polarity to a material remaining as a result of the charge imbalance becomes smaller than the ratio of the colored form of the material remaining as a result of the charge imbalance. For example, when the coloring of the EC element is started from the charge imbalance state in which the colored form of the cathodic EC material remains, the ratio of the colored form resulting from the anode material becomes smaller than the ratio of the colored form resulting from the cathode material as compared to a state in which the charge imbalance does not occur.
As a result, the actual absorption spectrum of the EC element changes from an absorption spectrum assumed at the time of its design, and the change appears as a change in absorption color of the EC element. Accordingly, the foregoing situation is not preferred. In U.S. Pat. No. 6,188,505, at the time of a decoloring operation by the charge imbalance, each of the redox buffers accepts the charge of the remaining oxidized form of the anode material or the remaining reduced form of the cathode material, and hence suppresses the remaining of the colored form of one of the anode and cathode polarities. However, the charge imbalance itself between the display electrodes is not corrected, and hence the shift of the ratio of the colored form of the anodic EC material to the colored form of the cathodic EC material is not corrected. In other words, merely a situation in which a color is observed at the time of the decoloring operation when the charge imbalance between the display electrodes occurs is suppressed, and at the time of a coloring operation, a colored state in which the ratio between the anode material and the cathode material changes from the original one owing to the charge imbalance appears.